Do you mean the bit that flaps the wiper back and forth? It's the windshield wiper motor. You could also mean the windshield wiper control, on the dash, that regulates the air flow to the motor. If you mean a pump, you could mean the air pump thingy that is inside the washer bottle that holds the fluid. It has an air regulator control on the dash too. So you could mean about 6 different things... They all run off air pressure.

Mine says "Air Push" on it. It wasn't the air line that broke, but the plate with the threads in it that the line screws into that broke. Im trying to upload pictures from my phone that shows it, but having trouble.

That link is to my windshield motor picture. I couldn't get it to upload here? It is a blurry picture, but it you look closely to a copper line just to the left of the yellow line. you will see what broke.

looks like the fitting broke off the motor hard to tell from photo. You need a manual on your bus (http://www.coachinfo.com/AllAboutBuses/GMC_Info.html or ebay or search the web),so you wont pull out all your hair trying to fix $tuff.You can also replace them with electric ones for a small fortune or look in wrecking yards to fabricate one out of a truck.

There is nothing wrong with air wipers they have a torque and speed adjustment setting screw once you figure the adjustments out and make sure the pressure regulator is set right they do a good job like on some Eagle buses they can even have intermittent delay speed